The present invention relates generally to controls and control methods for synchronizing a gas turbine-driven AC generator to an external AC power system and, more particularly, to control methods and control systems operable following gas turbine startup to guide the gas turbine-driven generator from a speed just below synchronous speed to proper speed and phase angle for synchronization in minimal time, with minimal fuel expenditure, and with minimal thermal stress on the gas turbine.
It is well known that if the phases and frequencies of a generator and an AC power system are not closely matched at the time a circuit breaker closes to connect the generator to the electrical power system, very large transfers of energy occur between the electrical power system and the generator. The resulting high electric current may damage the stator windings of the generator. Moreover, the accompanying transient torque on the generator shaft may reach values up to twenty times the design torque and cause failure of the shaft. Further, the stability of the external power system itself can be adversely affected. Thus, it is necessary for the generator to be synchronized to the power system before the generator is connected to the power system.
Particularly for a gas turbine-driven generator, it is also important that a minimal amount of time be taken to synchronize the generator to the power system. Gas turbine power plants are most often used to meet peak load demands in a power system. For this reason, they are started and stopped often. The amount of time required to start a gas turbine, synchronize it to the power system, and fully load it is important to power system operators and dispatchers. A fast starting gas turbine power plant allows quick response in emergency conditions in a power system, and keeps down operating costs.
It has long been desired to decrease the severity of the temperature cycle of the hot-gas-path parts of a gas turbine during start up. The gas turbine hot-gas-path temperature increases steadily as it is driven by its starting means and then starts to accelerate under its own power. As turbine speed increases, its efficiency increases, its acceleration increases, and the hot-gas-path temperature drops. With a turbine-generator power plant, it has been necessary to limit turbine acceleration, bringing it to zero (holding constant speed) as synchronous speed is reached, and then holding synchronization speed until the generator is synchronized to the power system. This results in a further drop in the hot-gas-path temperature. Once synchronism is increased, the generator picks up electrical load, and the turbine hot-gas-path temperature climbs once again.
It is desirable to synchronize a gas turbine-driven generator without having to hold synchronous speed, although this desire is not realized in conventional synchronizers. When realized, advantageously the acceleration limit during startup can be increased, the drop in the hot-gas-path temperature is less severe, and the generator can quickly pick up load.
At this point, it is important to note the distinction between gas turbine startup controllers and gas turbine synchronization controllers with which the present invention is concerned, although both types may well be combined in a single comprehensive control system. The function of a startup controller is to properly sequence and operate the gas turbine from an at-rest condition up to approximately 95% of synchronous speed. A startup controller typically sequences the gas turbine through stages of initial cranking, applying fuel and ignition when firing speed is reached, and thereafter allowing the gas turbine to accelerate while closely monitoring turbine operating parameters to accelerate the turbine as fast as possible without damage. It is particularly important to control the rate of temperature rise of turbine hot gas path parts so as to minimize undue stress. The startup controller relinquishes control to the separate synchronization controller when the gas turbine-driven generator is operating at nearly synchronous speed, for example, 95% of synchronous speed. An example of an analog-type gas turbine control system including a startup controller is disclosed in commonly-assigned Loft et al U.S. Pat. No. 3,520,133. Various startup controllers employing digital computers have more recently been proposed with the same ultimate function, namely, to accelerate the gas turbine-driven generator up to near synchronous speed.
The actual synchronization is an entirely separate process. Traditionally, synchronization has been accomplished manually by a skilled operator employing a synchroscope or similar device which provides a visual indication of the phase difference and slip (frequency difference) between the generator and the power system. The operator makes manual corrections to raise or lower the speed set point as required. When the generator ultimately reaches a point of synchronization with the power system, the operator initiates a signal to close the circuit breaker. Since it takes a finite time for a circuit breaker to actually close due to mechanical inertia, a skilled operator anticipates the breaker closing time.
More recently, various forms of automatic synchronization controllers have been proposed. In general, these synchronization controllers take the place of a skilled operator and issue raise and lower commands as is appropriate to bring the generator into synchronization with the power system. The "raise" and "lower" commands are effective, respectively, to raise and lower the speed set point or target speed at which the gas turbine controller is attempting to maintain the gas turbine. By way of example, the following two commonly-assigned U.S. patents are identified for their disclosures of synchronization controllers which issue commands to "raise" or "lower" speed for the purpose of synchronizing a generator to a power system: Konrad Pat. No. 3,801,796 and Kleba et al Pat. No. 4,249,088. Other examples of such synchronizers are disclosed in the patents to Rubner et al No. 3,562,545, Schlicher et al No. 3,794,846, Reed No. 4,031,407 and Barrett et al No. 4,118,635.
Another form of synchronization controller is disclosed in Haley U.S. Pat. No. 3,892,978. Haley proposes a synchronization controller which operates following initial turbine acceleration beginning at an initializing point when a predetermined angular velocity has been reached. At this point, the Haley controller calculates what value of constant acceleration will cause the turbine and generator to be guided to an appropriate synchronization point, and then adjusts the fuel flow of the gas turbine to a rate which should result in the calculated constant acceleration.
In this connection, it may be noted that, at or near synchronous speed of a gas turbine-driven generator, fuel flow and acceleration are approximately directly related, although rotational velocity is also a factor. Thus, a constant rate of acceleration requires essentially a constant rate of fuel flow.
During turbine startup operations subsequent to the ignition stage, but prior to breaker closure, fuel flow is modulated in a range between a minimum startup fuel flow and a maximum startup fuel flow. The minimum fuel flow is the approximate minimum required to sustain gas turbine operation without flame-out. A typical value of minimum startup fuel flow is 23% of maximum capacity fuel flow. The maximum startup fuel flow is selected in view of turbine characteristics and in view of the requirement to supply sufficient power to accelerate the turbine, but without requiring the turbine to actually supply any significant amount of power to the power system. The maximum startup fuel flow in some cases is not directly specified, but rather, follows as a function of an acceleration limit or as a function of a rate of temperature rise limit. The maximum startup fuel flow may, for example, be 38% of maximum capacity fuel flow.
Ideally, circuit breaker closure upon synchronization occurs when generator speed is slightly in excess of power system frequency such that, upon breaker closure, power flow is slightly positive. I.e., the generator goes on-line supplying power at approximately 5% of rated capacity. Thereafter, fuel flow to the turbine is increased beyond the maximum startup fuel flow rate, and the gas turbine-driven generator is allowed to gradually assume more load consistent with avoiding undue thermal stress. Optimally, generator frequency at the moment of breaker closure is within the range of from 0.2% to 0.5% greater than the power system frequency, with a phase difference of 0.degree..
In the art of synchronizing oncoming generators to power systems, the instantaneous operating points of the generator are plotted on a phase-frequency difference plane wherein the X or horizontal axis represents phase difference between the generator voltage and the power system voltage, and the Y or vertical axis represents frequency difference (known as slip) between the generator voltage and the power system voltage. During the synchronization process, a trajectory can be plotted in the phase-frequency difference plane, providing a convenient mechanism for visualization. For constant values of acceleration, trajectories approximate parabolas opening either to the left or to the right. For a positive value of acceleration, the resultant parabola opens up to the right. For a negative value of acceleration (i.e. deceleration), the parabola opens to the left.
Optimal synchronization points may be readily defined on the phase-frequency difference plane, and an analysis of the predicted or actual generator trajectory may then be made to determine or cause the trajectory to pass through or sufficiently near an optimal synchronization point.